Eccentric brahmins and charming talk on Maine's grandly desirably Mount Desert Island--in a mystery debut which, unfortunately, hardly stops talking long enough to get around to such mundane matters as plot, detection, or suspense. It's summer, in the late 1960s. The Worthington clan has gathered at The Bungalow on Schooner Cove: matriarch Elizabeth W., famed for her enamel-on-copper landscapes; sedate daughter Sarah, with genial husband Hill Halstead (a Boston lawyer) and kids; wild, nudistically inclined daughter Isabella; Mrs. W.'s jaunty spinster-sister Emily; and Mrs. W.'s beauteous new temporary secretary Jane Lamb, who has brought along her excessively precocious daughter Elizabeth. Much chat ensues, mostly about newcomers Jane and Elizabeth. There are visits with ""Natives"" and neighbors--including rich widow Mrs. Parker, who is hated for assorted good reasons by several characters. But Mrs. P. doesn't turn up dead until the novel's last quarter--which leaves just a few pages for super-child Elizabeth to finger the culprit. (A thoroughly ho-hum mystery in any case.) And, also in the crammed last pages, there's a revelation about Jane and Elizabeth that belongs to some other, fluffier genre. Morison shows a real talent here--for erudite, socio-comic description (""She looked as the mother of the Gracchi might have looked if attired in a Liberty print shirtwaist dress""), for WASPish repartee. But before attempting some promised sequels (with wee Elizabeth as sleuth), she'd best study up on basic plot construction.